It is known to industrially manufacture hollow bodies of a thermoplastic material by extruding a tubular preform of this material through an annular die, enclosing this preform in the interior of a mold and blowing air under pressure into the interior of this preform to inflate it and cause the plastic material to contact the walls of the mold, which are cooled so that at the contact of which the plastic material solidifies.
The mold that is utilized generally comprises two half-molds between which the preform is extruded. The "plug" which passes from the mold at its lower portion and which is constituted by the excess of plastic material, serves generally for the removal from the mold of the hollow body, thus obtained, by means of automatic pincers. These advance the hollow body to finishing stations where, among other operations, the plug is removed.
This mode of manufacture has a number of disadvantages particularly when, for reasons imposed by their ultimate utilization (filling, automatic handling etc.), the hollow body products must have a base with a very pronounced concavity.
These disadvantages which will be described in greater detail later, with reference to the annexed drawings, can be summarized as follows.
If the pincers seize the plug while the mold is still closed, the displacement of the mold-halves perpendicularly to the plane of the mold joint at the time of opening of the mold has the effect of enforcing an ascending movement of the hollow body by an amount equal to the concavity of the bottom. Since the body is held by the pincers through the intermediary of the plug, there results either a deformation of the bottom or a premature tearing and separation of the plug.
If to avoid this disadvantage, the pincers only seize the plug at the end of the course of opening of the half-molds, the hollow body is no longer supported in the plane corresponding to the plane of the mold joint and there is the risk that the body will be caught and displaced by one or the other of the half-molds. In addition, the pincers must seize the plug which is not properly guided and already cooled, this substantially affecting the regularity of the subsequent steps of the process.